1. Field of the Invention
The present invention relates to electronic switching power supplies and specifically to the integrated circuit devices used in switching power supplies.
2. Description of the Prior Art
The cost to manufacture a power supply is a principal concern in selecting power supply types to be used in particular applications and the components selected to construct them. Switching power supplies have become cost competitive with much simpler linear power supplies because integrated circuit (IC) technology has advanced such that a majority of the complex switch mode circuits can be integrated on a single chip. As a rule of thumb, the fewer necessary pins on an IC package the lower will be the cost. Fewer support components and less expensive peripheral components also will reduce the overall cost of a power supply.
FIG. 1 illustrates a prior art power supply 10 that includes a full-wave bridge rectifier 12, a transformer 14 with a primary winding 16 and a pair of secondary windings 18 and 20, a switched mode power supply chip 22, a plurality of filter capacitors 24-27, a pair of diodes 28 and 30, and a high-voltage high-wattage resistor 32. The power to operate chip 22 is produced by secondary winding 20, diode 30 and capacitor 27 and is referred to as .sup.V bias. However, at power start-up, chip 22 will be without power because primary winding 16 will be open and no voltage will be induced into secondary winding 20 because chip 22 is not switching. To initiate such switching, a point with high-voltage DC is tapped on bridge rectifier 12 by resistor 32 and the current is filtered by capacitor 25. This tap will supply just enough current through resistor 32 to start chip 22. Unfortunately, resistor 32 must be a high-wattage type because significant amounts of power can be dissipated in the process of dropping the high voltage of the line to the low voltage required by chip 22. The waste of power is continuous, even after Vbias is available. This problem is particularly severe when input voltage range is wide (e.g., 80-275 VAC for universal operation). Given worst case design rules, resistor 32 has to be designed to provide the required start up current for the lowest expected voltage. Thus, at the highest expected voltage, resistor 32 must dissipate all the more power. To compound the problem, such high-wattage resistor types require extra space and air circulation.
FIG. 2 illustrates a prior art power supply 50 that is similar to power supply 10, except resistor 32 has been eliminated and chip 22 is replaced by a switched mode power supply chip 52. A voltage regulator internal to chip 52 allows the elimination of resistor 32 and includes a high voltage pre-regulator transistor 54, a series-pass transistor 56 and an undervoltage comparator 58. A resistor 60 biases transistor 54 on and during initial power-up, transistor 54 will pull transistor 56 up and supply power to a pulse width modulator (PWM) 62 that switches on a power output transistor 64. A voltage develops across primary winding 16 and induces a voltage in secondary winding 20 which supplies .sup.V bias. With .sup.V bias being supplied, comparator 58 operates to maintain transistor 54 off and no further high voltage power is required. Turning off high voltage pre-regulator 54 will save power after start up, but such a function is more expensive to implement, as it requires a high voltage transistor and extra pin with high voltage safety spacing.
The typical prior art high voltage transistor used in the pre-regulator of switch mode power supply chips is usually a relatively small device. The transistor's immunity to line transients is therefore limited. Thus, the pin associated with the transistor becomes a limiting factor for electrical static discharge (ESD) and safe operating area (SOA) rating of the switching regulator chip.